1. Field of the Invention
The present invention relates to an offset adjusting circuit for an optical disc and an offset adjusting method.
2. Description of the Related Art
Generally, in an optical disc playback (and recording) apparatus (hereinafter referred to as an “optical disc apparatus”), after amplifying two output signals from an optical pickup, by a differential operational amplifier taking the difference between intensities of both the signals, a tracking error signal TE and a focus error signal FE for the optical disc are obtained. By controlling based on these error signals, a servo control system can make one spot of a light beam follow a track of the optical disc to irradiate precisely.
To date, the configuration of an error signal generating circuit for obtaining the tracking error signal TE and the focus error signal FE has included two chips: an IC chip (RF signal processing circuit) engaging in analog processing at the front stage and an IC chip (servo processing circuit) engaging in digital processing at the rear stage. That is, the analog processing system IC chip, after amplifying two output signals from the optical pickup, takes the difference between intensities of both the signals by a differential operational amplifier, and outputs the focus error signal and the tracking error signal to the digital processing system IC chip.
Then, the digital processing system IC chip converts the obtained focus error signal and tracking error signal into digital signals by an A/D converter and generates signals for adjusting offsets by an appropriate control logic in a digital signal processing section.
These offset adjusting signals are fed back to the analog processing system IC chip at the front stage so that the focus error signal and tracking error signal having the offset cancelled out are obtained (See, for example, Japanese Patent Application Laid-open Publication No. 10 105997 (FIG. 1)).
Specifically, as shown in FIG. 5, the optical disc apparatus irradiates an optical disc 105 with one spot of a light beam. First and second photo-detectors of an optical pickup receive reflected light from this optical disc 105. A servo signal processing circuit 140 cancels out the offsets to resolve the degradation of accuracy in focusing servo and tracking servo of the objective lens of the optical pickup 200.
A playback system PB of this optical disc apparatus will be explained. An RF signal processing circuit 110 supplies a combined signal having its offset cancelled out to a decoder 120. The decoder 120 performs processing such as de-interleave processing, decoding for error correction, EFM demodulation, and the like on the combined signal and supplies reproduced data to a memory 121.
The memory 121 is controlled in terms of write-in and read-out of data by a system controller (microcomputer) 150, and the reproduced data is written therein from the decoder 120. Also, the reproduced data is consecutively read out from the memory 121 at a constant bit rate. The reproduced data consecutively read out from the memory 121 is supplied to a decoder 122. When the reproduced data is compressed data, this decoder 122 decompresses the data to, for example, four times its size. Digital data from the decoder 122 is supplied to a D/A converter 123 to be converted into an analog signal, which is drawn to the outside through an output terminal 124. Then, as shown in, for example, FIG. 1 of the above-mentioned Japanese Patent Laid-open Publication, the offset adjuster of an RF signal processing circuit 2 at the front stage in a servo signal processing apparatus 1 adjusts the offset based on an offset adjustment amount calculated in a DSP (digital signal processor) of a servo processing circuit 90 at the rear stage.
Note that a recording system REC, as known well, is realized by constituents indicated by, for example, reference numerals 112 through 118 of FIG. 5.
In the above-mentioned related art, the offset is adjusted by calculating an offset adjustment amount on the servo processing circuit side at the rear stage. That is, the offset adjuster of the RF signal processing circuit 110 at the front stage does not take a measure against the offset on its own. Accordingly, if variations occur in the intensity of the reflected light because the specifications or the like of the optical pickup are different or the type of optical disc is different, the output signals from the optical pickup themselves cannot be accurately adjusted in terms of offsets. As a result, the error signals are generated based on the output signals whose offsets have not been accurately adjusted. Hence, at the stage where a differential operational amplifier takes the difference in intensity between two output signals of the optical pickup to generate an error signal, due to insufficient adjustment of the offset, the two output signals deviate from the dynamic range of a processing system including the differential operational amplifier, so that the waveforms thereof become distorted. Thus, an appropriate error signal cannot be generated, thereby causing the degradation of servo control based on the error signal.